Reciprocating linear hydraulic motors

ABSTRACT

Supplied hydraulic fluid pressure overcomes the force of a spring and moves a valve member, which is biased towards a closed position by the spring, into an open position, opening a passageway to admit the hydraulic fluid to a radial pressure surface on a linear piston. The hydraulic fluid pressure acting on the piston moves the piston against the spring, compressing the spring. When the piston reaches its retracted position hydraulic fluid pressure is added to the spring pressure causing the valve member to move downwardly into a position in which the fluid pressure is released from the pressure surface on the piston, enabling stored energy in the spring to forcibly drive the piston through a power stroke.

il'iiite Patent [191 Wohlwend RECIPROCATING LINEAR HYDRAULIC MOTORS [76]Inventor: Maurice Wohlwend, 5001 South 112th Street, Seattle, Wash.98178 [22] Filed: June 2, 1971 [21] Appl. No.: 149,217

92/134,173/134 [51] Int. Cl B25d 9/00 [58] Field of Search 173/119, 120,134,

45 June 19, 19

8/1969 James 173/119 2/1972 Coyne 173/119 [5 7] ABSTRACT Suppliedhydraulic fluid pressure overcomes the force of a spring and moves avalve member, which is biased towards a closed position by the spring,into an open position, opening a passageway to admit the hydraulic fluidto a radial pressure surface on a linear piston. The hydraulic fluidpressure acting on the piston moves the piston against the spring,compressing the spring. When the piston reaches its retracted positionhydraulic fluid pressure is added to the spring pressure causing thevalve member to move downwardly into a position in which the fluidpressure is released from the pressure surface on the piston, enablingstored energy in the spring to forcibly drive the piston through a powerstroke.

33 Claims, 19 Drawing Figures Sim 1 N 5 PAINTED- ,MJKMMJ PAIENIED m wBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to reciprocating linear hydraulic motors, and more particularlyto such motors of a type including a linear piston or hammer which isdriven through its power stroke by energy stored in a gaseous ormechanical spring, and in which hydraulic fluid pressure is employed forretracting the piston and storing energy in the spring.

2. Description of the Prior Art The body of technology relating toreciprocating hydraulic linear motors is quite large and includes manydifferent ways of moving a piston or working member through a powerstroke. It is known to use hydraulic fluid pressure for both driving andretracting the piston or working member. Examples of this type of motorare shown by U. S. Pat. No. 3,4l1,592 granted on Nov. 19, 1968 to RogerMontabert, and by U. S. Pat. No. 3,339,644 granted Sept. 5, I967 toErnest F. Klessig. It is also known to move the piston in one directionby the application of hydraulic fluid pressure pulses to the piston, andthen move itin the opposite direction by use of a spring. U. S. Pat. No.3,456,741 granted July 22, 1969 to David Richard James discloses a motorof this type in which the piston is driven through a forward powerstroke by the springand is returned or retracted by fluid imposedpressure pulses, to store energy in the spring, The present inventionrelatesto the provision of improved motors of this. general type,characterized by a simplicity of overall design a minimum of parts,relatively low manufacturing costs, ease of assembly, disassembly, andmaintenance, and reliability throughout a long life of repetitious use.

SUMMARY OF THE INVENTION Motors of the present inventioncharacteristically comprise a fluid or mechanical spring for driving apiston type power element or hammer through its power stroke, and asimple system for delivering hydraulic fluid pressure 'againstrthepiston for the purpose of retracting it and storing energy in the springand at this time exhausting fluid-from the motor housing. In preferredform, the motors of this invention include a sin? gle reciprocatinglinear valve member for controlling the delivery of fluid pressure tothe piston. All valving functions are performed by this member and thepiston in conjunction with flow passageway in the motor housing, thepiston and the valve member. The valvemem-. her is biased towards aby-pass position by the fluid or mechanical spring. Inflowing hydraulicfluid is first directed against a transverse or radial surface on thevalve member, to exert a force on the valve member of sufiicientmagnitude to overcome the spring force and move the valve member into asupply position. This communicates the inflowing hydraulic fluid with atransverse or radial pressure surface on the piston, resulting in thehydraulic fluid exerting a force on the pis-v ton in opposition to thespring force, causing retraction of the piston. While retracting thepiston forces the used hydraulic fluid to be exhausted from the motorhousing and also stores energy. in the spring. when the piston reachesits fully retracted position passageways are opened through whichhydraulic fluid pressure is delivered to a transverse or radial pressuresurface on the valve member facing the spring, so that the hydraulicfluid pressure is added to the spring force. Together the spring andhydraulic fluid exert a force on the valve member of sufficientmagnitude to return the valve member to its by-pass position. When thevalve member is in its by-pass position inflow of additional hydraulicfluid is prevented and the fluid pressure is removed from the pressuresurface on the piston, freeing the spring to drive the piston throughits power stroke. During the power stroke this fluid is moved by thepiston into an upper expansible chamber bounded in part by a surface onthe piston (i.e. the moving boundary of the chamber) which is directedoppositely from said pressure surface. Then, during piston retractionthe piston moves such fluid] out of this chamber and the motor casingback to the supply pump.

An important feature of the invention relates to the provision of meansfor isolating the piston from the spring force after the piston deliversa blow to an impact member or the like but before such hammer strikesany portion of the casing in which it is housed, so that the piston isnot driven hard against a portion of the casing but rather is allowed tocoast to a stop before reversing its direction of travel.

Another important feature involves the provision of a body of cushioningfluid to function as a stop for the piston at the end of its powerstroke, so that it never impacts directly against the housing. 7

Other features of the invention will be apparent from the followingdescription and accompanying sheets of drawing. A

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a handtype impact tool incorporating a reciprocating linear hydraulic motorembodying the invention;

FIG. 2 is an enlarged scale, fragmentary, axial sectional view of themotor, with some parts shown in side elevation, such view showing thepiston at rest and the valve member in its by-pass position;

FIG. 3 is a view like FIG. 2, but showing the control valve raised byincoming hydraulic fluid into its supply position;

FIG. 4 is a view like FIGS. 2 and 3, but showing the piston partiallyraised by the hydraulic fluid;

FIG. 5 is a view like FIGS. 2-4, but showing the piston raisedanadditional amount by the hydraulic fluid;

FIG. 6 is a view like FIGS. 2-5,. but showing the piston fully raisedinto its retracted position and the control valve member returned to itsbypass position;

FIG. 7 is a cross-sectional view taken through FIG. 2, substantiallyalong line 7--7 thereof;

FIG. 8 is a fragmentary axial sectional view taken substantially alongline 8-8 of FIG. 7;

FIG. 9 is a cross-sectional view taken substantially along line 9--9 ofFIG. 2;

FIG. 10 is another cross-sectional view taken substantially along line10--l0 of FIG. 2;

FIG. 11 is another cross-sectional view taken substantially along line11-11 of FIG. 2;

FIG. 12 is an axial sectional view of a modified form of motorcharacterized by a side placed valve element;

FIG. 13 is a cross-sectional view taken substantially along line 13-l3of FIG. 12;

FIG. 14 is a cross-sectional view taken substantially along line 14l4 ofFIG. 12;

FIG. 15 is a cross-sectional view taken substantially along line l5-15of FIG. 12;

FIG. 16 is a cross-sectional view taken substantially along line 16-16of FIG. 12;

FIG. 17 is a cross-sectional view taken substantially along line 17-17of FIG. 12;

FIG. 18 is a cross-sectional view taken substantially along line 1818 ofFIG. 12; and

FIG. 19 is a cross-sectional view taken substantially along line l919 ofFIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a preferred typeof utilization device for the motor of this invention. It is a hand heldbreaker tool T adapted to be an accessory for an item of powerequipment, such as an earth working machine or the like, of a type whichincludes a hydraulic power system. When needed, the breaker tool ismerely connected into the hydraulic system of such machine to be poweredthereby, making it unnecessary for the workman to bring a moreconventional pneumatic breaker tool to the work site. Of course, it isto be understood that the motor of this invention has general utilityand can be used wherever a linear motor requiring its capabilities isneeded.

Referring now to FIGS. 2-11, in preferred form the motor of thisinvention is shown to comprise an elongated sectional casing orhousing-l formed to include an elongated inner cavity 12. A hammer orpiston 14 is supported for axial reciprocating movement within thecavity 12. The upper portion 16 of cavity 12 is charged with a gas (e.g.nitrogen) under substantial pressure. This gas functions as a spring for(1) driving the piston 14 downwardly through its power stroke and (2)biasing a valve member towards a by-pass position, as will hereinafterbe explained in detail. I

Piston 14 has an intermediate portion 18 of a first diameter which ismachined to include a girth groove or annular channel 20. Piston 14 alsoincludes a smaller diameter striker portion 22 at its lower end and areduced diameter upper end portion 24 which receives the force of thegas spring. Striker portion 22 is snugly accommodated within a bore 26formed through a lower portion 28 of housing 10. A suitable seal 30(e.g. a chevron seal) is provided to seal against leakage between thesurfaces of the striker portion 22 and the bore wall 26.

The upper end portion 24 of piston 14 is relatively snugly receivedwithin a cup-like member 32. This member 32 comprises a transverse endwall 34 and a tubular side wall 36 extending axially from end wall 34 insurrounding relationship to the upper end portion 24 of piston 14. Thecompressed gas spring does not act directly on the piston part 24, butrather exerts itself on the transverse wall 34 and throughout most ofthe operating cycle wall 34 in turn transfers the force to the pistonpart 24.

The casing includes an internal annular wall 38 in which the upperportion of an axially reciprocating sleeve valve member 40 is received.The tubular side wall 36 of cup member 32 is slidably received in anupper bearing portion 42 of valve member 40. The bearing 42 is bothinternally and exteriorly grooved to receive sealing rings 46. Inpreferred form the bearing 42 is separate from the rest of valve member40 but the two pieces move together and function as one.

The lower end portion of sleeve valve member 40 snugly surroundinglyengages the intermediate portion 18 of piston 14. The lower end portionof member is snugly received within a casing part 48 and is externallygrooved to receive a sealing ring 50 for sealing against fluid leakagebetween the surfaces of the parts 40, 48.

Inlet passageways 52 for inflowing hydraulic fluid are formedlongitudinally through a side wall portion of the housing 10 from anoff-on control valve 54 (FIG. 1) at the upper end of the housing 10 downto an annular chamber 56 formed in the lower portion 28 of casing 10generally below the lower end surface 58 of sleeve valve member 40. Anannular chamber 60 is defined radially between the annular lower endportion 62 of sleeve valve member 40 and the striker portion 22 ofpiston 14, and axially between a radial surface 41 on the lower endportion 28 of housing 10 and an opposing radial surface 43 on theintermediate portion 18 of piston 14. Herein this surface 43 will bereferred to as the pressure surface. The boundary to chamber 60 providedby housing portion 28 is a transverse end wall 41 against which thetransverse lower end surface 58 of valve member 40 is seated when thevalve member 40 is in the position shown by FIG. 2 of the drawing (i.e.in its by-pass position).

A plurality of axial passageways 66 are formed in the intermediateportion 18 of piston 14 for communicating chamber 60 with girth groove20.

When the piston 14 and the valve member 40 are in l the positions shownby FIG. 2 hydraulic fluid flowing into chamber 56 first contacts endsurface 58 of valve member 40. At this time the opposite end surface ofvalve member 40 is subjected to the downwardly biasing force of the gasspring, as is the piston 14 through cup wall 34. The inflowing hydraulicfluid in chamber 56 exerts a force on the lower end surface 58sufficient to overcome the gas spring force exerted on upper end surface70. As a result, the inflowing hydraulic fluid forces sleeve valvemember 40 upwardly until the annular shoulder 72 thereon contacts theannular stop 74. The inflowing hydraulic fluid now flows from chamber 56through chamber 60, then through axial passageways 66 into the girthgroove 20. It also exerts an axially upwardly directed force against thepressure surface 43. This subjects the piston 14 to an upward force of amagnitude sufficient to overcome the downwardly biasing force of thepressure spring acting on piston 14 through wall 34. This causes thepiston 14 to be moved upwardly from the position shown by FIG. 3,through the intermediate position shown by FIG. 4, and into the positionshown by FIG. 5.

In FIG.4 the annular chamber 20 is approaching registry with a pluralityof radial ports 76 extending through a side wall portion of sleeve valvemember 40. Ports 76 communicate the inside of member 40 with an annularexpansion chamber 78 formed by and axially between opposing radialsurfaces on the inner wall 38 and on a piston ring 79 carried by thesleeve valve 40. When the ports 76 come into registry with chamber 20(FIG. 5) some of the hydraulic fluid flows through the ports 76 andexerts a downwardly acting force on the annular piston surface 80. Thesum of the fluid spring force acting on surface and the hydraulic fluidforce acting on piston surface 80 is larger than the force exerted byhydraulic fluid on valve member surfaces 58 and 72, resulting in thevalve member 40 being moved downwardly into its by-pass position. Thismoves the annular shoulder 72 of valve member 40 out from contact withthe stop surface 74 and opens an annular passageway 82. The seated lowerend portion 62 of valve member 40 stops (i.e. blocks) inflow ofadditional hydraulic fluid into the chamber 60. The fluid already inchamber 60 is exhausted therefrom through the radial ports 84 into theannular space 86 and from the annular space 86 through the newly createdpassageway 82 into the annular by-pass chamber 88. A plurality of axialports 90 communicate chamber 88 with a collector chamber 92 and returnpassageways 94 extend from chamber 92 up to conduit means extending backto the pump (not shown).

When chamber 60 comes into communication with by-pass chamber 88 thepressure exerted by the by draulic fluid on pressure surface 43 isreleased, allowing the stored energy of the gas spring to be releasedfor driving the piston 14 downwardly through its power stroke. Theexpansible chamber 104 above intermediate portion 18 of piston 14changes volume at substantially the same rate as chamber 60. During thepower stroke the fluid removed from pressure surface 43 is merelytransferred from chamber 60 through passageways 82, 108, 106 intochamber 104.

The gas does not act directly on the piston 14, but rather acts on wall34 which transmits the driving force to the piston 14 until theperipheral lip 96 of member 34 contacts the annular stop member 98.Member 98 is preferably a bellville type spring so that it will cushionthe stop of end wall 34. Wall 34 seats against spring 98 after thestriker portion 22 of piston 14 has struck the anvil of the breaker tool100 but while piston surface 102 is still spaced from wall 64. When wall34 is seated on the spring 98 the force of the fluid spring is removedfrom the upper end portion of the piston 14 and the piston 14 is free tocoast to a stop. Also, as best shown in FIG. 2, a cushion of hydraulicfluid always remains in the chamber 60 and provides a fluid stop for thepiston during the driving stroke.

The gas spring serves to cushion the piston 14 at the end of its upwardtravel at the end of the return stroke. Annular lip 105 prevents cupmember 32 from traveling up into gas chamber 16 in the event it getsaway from piston 14.

When the piston 14 moves downwardly it communicates chamber 78 with thechamber 88 via radial ports 76, annular chamber 104, and radial ports106 and 108. The chamber 78 remains exhausted until the piston 14 isagain moved upwardly a distance sufficient to place the seal ring 19above the radial ports 76 (FIG. 5). Seal ring 19 is preferably a splitmetal sealing ring like the type used on the pistons of internalcombustion engines. This is so it willnot be distorted by fluid pressurewhen it passes across ports 76. During retraction of the piston 14 thereis a positive displacement of the fluid from chamber 104 out throughports 106, s, chamber 88, ports 90, chamber 92, and passageways 94, backto the supply pump (n ot shown).

One or more surge chambers may be provided in the chamber 88. In thepreferred embodiment these chambers are shown to be inwardly bounded bya flexible wall 112 and outwardly bounded by a solid wall 114 shown toclamp an annular portion of the flexible wall 1 12 between it and aportion of the casing 10, immediately bounding the chamber 1 10. Air orsome other gas is trapped inside each chamber 110. When a pressure surgeoccurs within the chamber 88 it merely forces the flexible walls 112into the chambers 110 and in this manner is dampened.

In operation, hydraulic fluid supplied to annular chamber 56 exertsitself on the lower end surface 58 of valve member 40 and exerts anupwardly directed endwise force on the valve member 40 of sufficientmagnitude to overcome the downwardly directed force of the gas springacting on surface 70. The valve member 40 is moved upwardly by thehydraulic fluid pressure until the annular shoulder 72 contacts and isstopped by the annular stop 74 (FIG.v 3). The hydraulic fluid then movesfrom space 56 under valve member portion 62, into chamber 60, and fromchamber 60 through the passageways 66 into annular groove 20. In chamber20 the fluid exerts an upwardly directed force on surface 43, causingthe piston 14 to move upwardly relative to reaction surface 41 onhousing 18 below the piston surface 43, and in opposition to the gasspring force exerted on wall 34 and by it against the upper end ofpiston 14.

Once sealing ring 19 passes above the radial ports 76 (FIG. 5) the fluidin chamber 20 flows therefrom radially out-wardly through the ports 76and into the expansible chamber 78 above the annular piston ring 79. Thehydraulic fluid force on piston surface 80, and the gas spring force onsurface 70, act together to force the valve member 40 endwise downwardlyuntil the lower surface 58 is again seated on housing surface 41 (FIG.6). Such movement of valve member 40 opens annular passageway 82,communicating chamber 60 with the by-pass chamber 88. In that manner theupwardly directed force is removed from the piston 14. Then, the storedenergy in the gas piston exerts itself on wall 34 and through wall 34onto the upper portion 24 of the piston 14, and forces the piston 14downwardly through a power stroke. As shown by FIG. 6, when valve member40 is seated and the piston 14 is retracted fluid is trapped in chamber78. As a result valve member 40 is held in its seated position until thepiston 14 has been driven downwardly.

In the illustrated embodiment the piston 14 is forced downwardly by thecompressed gas until the lower end of its striker portion 22 hits theupper end (not shown) of a moil point (FIG. 1). This contact occurswhile piston surface 43 is still spaced a sufficient distance abovehousing surface 41. The force of the compressed gas is applied on thepiston 14 until the flange 96 on cup member 32 makes contact with and isstopped by the annular spring 98. This seating of flange 96 on spring 98also occurs at a point of time when piston surface 43 is spaced upwardlyfrom housing surface 41. As a result, the piston 14 is not forciblydriven by the compressed gas against a portion of the housing. Rather,once cup member 32 seats itself on spring stop 98, the piston 14 coaststo a stop before changing its direction of travel. As best shown by FIG.2, the piston surface 43 never does contact housing surface 41 becausean annular body of the incompressible hydraulic fluid becomes trappedaxially between piston surface 43 and housing surface 41 when the piston14 is in the position shown by FIG. 2. This annular body of hydraulicfluid functions as a fluid stop for the piston 14.

The piston 14 is cushioned in the upper direction by the compressed gasspring and in the downward direction by the hydraulic fluid in chamber60.

Reference is now made to the secondary embodiment of the invention shownby FIGS. 12-19. This embodiment diflers from the embodiment of FIGS.2-11 in that its reciprocating valve member 116 is spaced laterally fromthe piston 118. However, the operation of this form is almost the same,as will presently be seen.

In the embodiment shown by FIGS. 12-19 the incoming hydraulic fluidtravels from suitable valving (not shown) atop the housing 120 downthrough a longitudinal inlet passageway 122, into an annular chamber124, and thence through radial ports 126 into chamber 128 located belowthe valve member 116. It then presses upwardly against the lower endsurface 130 of valve member 116, forcing valve member 116 upwardly offfrom its seat 118. This communicates chamber 128 with an annularpassageway 134 which is connected by ports 136 and a passageway 138 withan annular chamber 140 defined axially between a lower radial surface142 on piston 118 and an opposing radial surface 144 on housing 120.

The hydraulic fluid flows from annular chamber 140 through axialpassageways 143 into annular girth chamber 146 formed in the piston 1 18closely below a metal sealing ring 148 which surrounds a midportion ofthe piston 118. The hydraulic fluid exerts an upwardly directed force onpiston surface 142, in opposition to a downward force provided by afluid spring within chamber 152. The hydraulic fluid lifts piston 118upwardly relative to reaction surface 144 until sealing ring 148 isabove the radial ports 154. At this point of time the shoulder 155 onvalve member 116 is against stop member 156. The hydraulic fluid flowsfrom chamber 146 through the ports 154 into the expansible chamber 158above radial surface 160 on valve member 116. The hydraulic fluid exertsa force on surface 160 which is added to the force exerted by the gas inchamber 152 on upper end surface 162, and together I they move the valvemember 116 downwardly until the surface 130 is again seated on thesurface 118. When this happens chamber 140 is communicated throughannular passageway 163 to a housing chamber 166 and from there into thehollow body of valve member 116 through radial ports 168. The fluidemerges from an upper set of radial ports 170 into an upper passageway172 and thence into chamber 173. Once the fluid pressure is relievedfrom piston surface 142 the gas spring acts to forcibly drive the piston118 downwardly through its power stroke. The piston 118 moves downwardlyuntil its striker portion 176 strikes a blow against the moil point 178.Shortly after this happens the annular flange 180 at the periphery ofend wall 82 on drive member 184 contacts and is stopped by annularspring 186. As in the earlier embodiment, this isolates the compressedgas spring from the piston 118 which then coasts to a stop against anannular body of hydraulic fluid trapped within the chamber 140.

In this embodiment the valve member 116 is provided with a sealing ring188 positioned to ride housing wall 190, and a smaller ring 192 arrangedto ride housing wall 194. The other sealing rings illustrated may bechevron type packings, for example.

As in the earlier form, when valve member 116 is seated and piston 118is retracted flow through ports out from chamber 158, through ports 154,and into chamber 172. I

The invention maybe embodied in other specific forms without departingfrom the spirit or central characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the fore- 154 is blockedand fluid is trapped within chamber 158 going description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore to be embraced therein.

What is claimed is: 1. A linear reciprocating motor comprising: ahousing including an elongated piston cavity defined in part byatransverse wall; an elongated piston in said cavity; spring meansexerting a force on said piston tending to drive it endwise through saidcavity towards said transverse wall; 7 hydraulic means for driving thepiston endwise through the cavity in the opposite direction, inopposition to the force of said spring means, with said spring meansbeing compressed by said piston as it so moves, said hydraulic meansincluding a pressure surface on said piston spaced axially from saidtransverse wall and in fluid communication therewith, and means fordelivering a hydraulic fluid to said transverse wall and said pressuresurface, including control valve means comprising a linear reciprocatingvalve member movable between a first position in which hydraulic fluidis admitted to the pressure surface and the transverse wall, for mov ingthe piston against the spring means and compressing the latter, and asecond position in which such hydraulic fluid is removed from saidpressure surface and flow of additional hydraulic fluid to said pressuresurface is blocked by said valve member, resulting in the force of thehydraulic fluid being removed from said piston and said piston beingforcibly moved by the spring means endwise through the cavity towardssaid transverse wall, said valve member having a first end portion whichis in communication with the inflowing hydraulic fluid and receives anendwise force therefrom tending to move said valve member towards itsfirst position, a second end portion, biasing force means acting on saidsecond end portion and tending to move the valve member towards itssecond position, with said inflowing hydraulic fluid by itself exertinga force on said valve member that is larger than then biasing force,said valve means further comprising means for directing some of thehydraulic fluid against the valve member, to exert a force on the valvemember acting in the same direction as the biasing force when the valvemember is in its first position, with the combined forces of saidbiasing force means and said hydraulic fluid being larger than the forceof said inflowing hydraulic fluid and being sufficient to move saidvalve member into its second position. i 2. A motor according to claim32, wherein the spring means is a body of a compressible fluid sealedwithin a chamber which is expansible towards the piston.

3. A linear reciprocating motor comprising: a housing including anelongated piston cavity defined in part by a transverse wall; anelongated piston in said cavity;

spring means exerting a force on said piston tending to drive it endwisethrough said cavity towards said transverse wall;

hydraulic means for driving the piston endwise through the cavity in theopposite direction, in opposition to the force of said spring means,with said spring means being compressed by said piston as it so moves,said hydraulic means including a pressure surface on said piston spacedaxially from said transverse wall and in fluid communication therewith,and means for delivering a hydraulic fluid to said transverse wall andsaid pressure surface, including control valve means comprising a valvemember movable between a first position in which hydraulic fluid isadmitted to the pressure surface and the transverse wall, for moving thepiston against the spring means and compressing the latter, and a secondposition in which such hydraulic fluid is removed from said pressuresurface and flow of additional hydraulic fluid to said pressure surfaceis blocked by said valve member, resulting in the force of the hydraulicfluid being removed from said piston and said piston being forciblymoved by the spring means endwise through the cavity towards saidtransverse wall, wherein said valve member is annular and surrounds aportion of said piston, said valve member includes a first end surfacewhich is seated against a portion of said transverse surface when thevalve member is in its second position, and a second end surface whichis subjected to the force of the spring means, such force tending tourge the valve member towards said transverse surface, and said housingincludes passageway means for delivering hydraulic fluid to the firstend surface of said valve member, for exerting a force on said first endsurface which moves the valve member from its second position into itsfirst position. i

4. A motor according to claim 3, wherein said housing includes aninternal tubular guide for said valve member in which said valve membermoves, means forming a bypass chamber which at least partially surroundsthe valve member, said valve member including side wall opening meansthrough which the hydraulic fluid flows from the piston pressure surfaceinto said by-pass chamber when the valve member is in its sec ondposition and the piston is traveling through its power stroke.

5. A motor according to claim 4, wherein said housing includes meansforming an internal annular seat generally at the entrance to saidexhaust chamber, and said valve member includes an exterior annularclosure portion which seats tightly against said seat when the valvemember is in its first position, and prevents hydraulic fluid fromflowing into the by-pass chamber so that it instead exerts a force onthe pressure surface of the piston.

6. A motor according to claim 5, wherein the tubular guide for saidvalve member includes means forming an expansible chamber with saidvalve member, in surrounding relationship to said valve member, and saidvalve member includes apiston ring portion defining a movable wall ofsaid chamber, and said valve member includes side wall opening meansthrough which the hydraulic fluid flows from the region of the pistonpressure surface into said expansible chamber when the valve member isin its first position and the piston is'retracted against the springmeans, whereby the hydraulic fluid exerts a force on said piston ringwhich in combination with the spring means force on the valve memberacts to move the valve member from its first position into its secondposition.

7. A motor according to claim 6, wherein said motor includes passagewaymeans for exhausting the fluid from the expansible chamber into theby-pass chamber when the valve member is in its second position and thepiston is moving through its power stroke.

8. A motor according to claim 7, wherein said housing includes aseparate cavity for said valve member which is offset from the pistoncavity, and said valve member includes a first end surface which isseated against a portion of said transverse surface when the valvemember is subjected to the force of the spring means, such force tendingto urge the valve member towards said transverse surface, and saidhousing includes passageway means for delivering hydraulic fluid to thefirst end surface of said valve member, for exerting a force on saidfirst end surface which moves the valve member from its second positioninto its first position.

9. A motor according to claim 8, wherein said housing includes meansforming a by-pass chamber which at least partially surrounds the valvemember, said valve member including side wall opening means throughwhich the hydraulic fluid from the piston pressure surface is exhaustedwhen the valve member is in its second position and the piston is movingthrough its power stroke.

10. A motor according to claim 9, wherein said housing includes meansforming an internal annular scat generally at the entrance to saidby-pass chamber, and said valve member includes an exterior annularclosure portion which seats tightly against said seat when the valvemember is in its first position, and prevents hy' draulic fluid fromflowing into the by-pass chamber so that it instead exerts a force onthe pressure surface of the piston.

11. A motor according to claim 10, wherein the cavity for said valvemember includes means forming an expansible chamber with said valvemember, in surrounding relationship to said valve member, and said valvemember includes a piston ring portion defining a movable wall of saidchamber, and said housing includes passageway means through which thehydraulic fluid flows from the region of the piston pressure surfaceinto said expansible chamber when the valve member is in its firstposition and the piston is retracted against the spring means, wherebythe hydraulic fluid exerts a force on said piston ring which incombination with the spring means force on the valve member acts to movethe valve member from its first position into its second position.

12. A motor according to claim 11, wherein said motor includespassageway means for exhausting the fluid from the expansible chamberwhen the valve member is in its second position and the piston has movedsubstantially through its power stroke.

13. A linear reciprocating motor comprising:

a housing including an elongated piston cavity defined in part by atransverse wall;

an elongated piston in said cavity;

spring means exerting a force on said piston tending to drive it endwisethrough said cavity towards said transverse wall;

hydraulic means for driving the piston endwise through the cavity in theopposite direction, in opposition to the force of said spring means,with said spring means being compressed by said piston as it so moves,said hydraulic means including a pressure surface on said piston spacedaxially from said transverse wall, and means for delivering a hydraulicfluid to said surface, including control valve means movable between afirst position inwhich the hydraulic fluid is admitted to the pressuresurface, for moving the piston against the spring means and compressingthe latter, and a second position in which the hydraulic fluid isexhausted from the pressure surface, resulting in the piston beingforcibly moved by the spring means endwise through the cavity towardssaid transverse wall; and

limit means for isolating the piston from the force of said spring meansbefore the piston reaches the transverse wall, so that the piston is notdriven hard against the transverse wall by the spring means but is freeto coast to a stop position as it approaches the transverse wall beforereversing its direction of travel.

14. A motor according to claim 13, wherein a fluid containing chamberexists axially between saidv transverse wall and said piston and thefluid therein provides a cushioning stop for said piston.

15. A motor according to claim 13, wherein the spring means is a body ofa compressible fluid sealed within a chamber which is expansible towardsthe piston.

16. A motor according to claim 15, wherein the limit means comprises abarrier for the fluid spring chamber and means for arresting movement ofsaid barrier independently of said piston, positioned so that thebarrier is stopped before the piston reaches said transverse wall andserves to isolate the spring force from the piston. i;

17. A motor according to claim 16, wherein a fluid containing chamberexists axially between said transverse wall and said piston and thefluid therein provides a cushioning stop for said piston.

18. A motor according to claim 16, wherein an axial wall means isconnected to said barrier and is in part at least tubular and outwardlybounds at least a portion of said piston.

19. A motor according to claim 18, wherein a bearing is radiallyreceived between said axial wall means and an inner side wall portion ofthe cavity for guiding said axial wall means.

- 20. A motor according to claim 15, wherein the stop member for saidwall means is a mechanical spring.

7 21. A motor according to claim 20, wherein an axial wall means isconnected to said barrier and is in part at least tubular and extendsaxially from said barrier in at least partial surrounding relationshipto the near end portion of the piston, and said mechanical spring isannular in shape and has a central opening forloosely receiving saidaxial wall means, and a bearing is radially received between said axialwall means and an inner side wall portion of'- the cavity, for guidingsaid axial wall means.

22. A motor according to claim 21, wherein a fluid containing chamberexists axially between said transverse wall and said piston and thefluid therein provides a cushioning stop for said piston.

23. A gas spring powered impact tool, comprising:

a linear impact member;

a reciprocating linear hammer in line with said member;

a compressed gas spring'in line with said hammer, for forcibly drivingsaid hammer endwise through a power stroke, into impacting contact withsaid member; and

a hammer return hydraulic system including means for alternatelydelivering hydraulic fluid pressure against the hammer for moving itendwise against the compressed gas spring for retracting the hammer andstoring energy in the compressed gas spring, and releasing saidhydraulic fluid from the hammer, so that the stored energy in thecompressed gas spring can drive the hammer through its power stroke. r

24. An impact tool according to claim 24, including means for trapping abody of the hydraulic fluid being a portion of the hammer and a portionof the housing to provide a fluid stop at the end of the power stroke.

25. A linear reciprocating motor comprising:

a housing including an elongated piston cavity defined in part by atransverse wall;

an elongated piston in said cavity;

spring means exerting a force on said piston tending to drive it endwisethrough said cavity towards said transverse wall;

hydraulic means for driving the piston endwise through the cavity in theopposite direction, in opposition to the force of said spring means,with said spring means being compressed by said piston as it so moves,said hydraulic means including a pressure surface on said piston spacedaxially from said transverse wall and in fluid communication therewith,and means for delivering a hydraulic fluid to said transverse wall andsaid pressure surface, including control valve means comprising a valvemember movable between a first position in which hydraulic fluid isadmitted to the pressure surface and the transverse wall, for moving thepiston against the spring means and compressing the latter, and a secondposition in which such hydraulic fluid is removed from said pressuresurface and flow of additional hydraulic fluid to said pressure surfaceis blocked by said valve'member, resulting in the force of the hydraulicfluid beingre'moved from said piston and said piston being forciblymoved by the spring means endwise through the cavity towards saidtransverse wall, and means in said housing defining an expansiblechamber for receiving some of the hydraulic fluid when the piston is atleast partially retracted against the spring, said means including amovable surface on said valve member, said movable surface on said valvemember being directed such that fluid in said chamber acting on saidmovable surface tends to urge the valve member towards its secondposition. 26. A motor according to claim 25, further comprising means insaid housing including a portion of said piston for trapping fluid insaid chamber, so that said fluid will hold the valve member in itssecond position,

until the piston has moved a predetermined distance an elongated pistonin said cavity;

spring means exerting a force on said piston tending to drive it endwisethrough said cavity towards said transverse wall;

hydraulic means for driving the piston endwise through the cavity in theoppositedirection, in op position to the force of said spring means,withsaid spring means being compressed by said piston as it so moves,said hydraulic means including a pressure surface on said piston spacedaxially from said transverse wall and in fluid communication therewith,andmeans for delivering a hydraulicfluid to said transverse'wall andsaid pressure surface, including controlvvalve means comprising a linearre ciprocating valve member movable between a first position in whichhydraulic fluid is admitted tothe pressuresurface and the transversewall, formovingthe piston against the spring meansand compressing thelatter, and a second position in which such hydraulic fluid is removedfrom said pressure surface and flow of additional hydraulic fluid tosaid pressure surface'isblocked by said valve member, resulting in theforce of the hydraulic fluid being removed from said piston and saidpiston being forcibly moved by the spring means endwise through thecavity towards said transverse wall, valvemember having a first endportion which is in communication with the'inflowing hydraulic fluid andreceives an endwise force therefrom tending, to move said valve membertowards its first position, and a second end portion subject to theforce of said spring means, with the force of said spring means tendingto move the valve member in the opposite direction towards its secondposition, with said inflowing hydraulic fluid by itself exerting: alarger force on said valve member and with said valve means furthercomprising means for directing some of the hydraulic fluid against thevalve member, to exert a force on the valve member acting in the samedirection as the force exerted by the spring means, when the valvemember is in its first position and the piston is against the springmeans and has compressed same, with the combined forces of said springmeans and said hydraulic'fluid.

being larger than the force of said inflowing hydraulic fluid and beingsufficient to move said valve member into its second position.

28. A linear reciprocating motor comprising:

a housing including an elongated piston cavity defined in part by atransverse wall;

an elongated piston in said cavity;

spring means exerting a force on said piston tending to drive it-endwisethrough saidcavity towards said transverse wall;

hydraulic means for driving the piston endwise through the cavity in theopposite direction, in opposition to the force of said spring means,with said spring means being compressed by said piston as it so moves,said hydraulic means including a pres sure surface on said piston spacedaxially from said, transverse wall and in fluid communication therewith,and means for delivering a hydraulic fluid to said transverse wall andsaid pressure surface, ineluding control valve means comprising a valvemember movable between a first position in which hydraulic fluid isadmitted to the pressure surface and the transverse wall, for moving thepiston against the spring means and compressing the latter, and asecondposition in which such hydraulic fluid is removed from saidpressure surface and flow of additional hydraulic fluid to said pressuresurface is blocked by said valve member, resulting in the force of thehydraulic fluid being removed from said piston and said piston beingforcibly moved by the springmeans endwise through the cavity towardssaid transverse wall, and means in said housing including a portion ofsaid piston for transferring the hydraulic fluid from said pressuresurface into an expansible chamber near the spring end of said pistonwhich chamber is bounded in part by a movable surface on said piston,said chamber including outlet means leading to exhaust, so that when thepiston is moved by new hydraulic fluid towards said spring the fluid insaid chamber is forced therefrom by the piston through the outlet meansto exhaust.

29. A gas spring powered impact tool, comprising:

a linear impact member;

a reciprocating linear hammer in line with said member; i

a compressed gas spring in line with said hammer, fo

forcibly driving said hammer endwise through a power stroke, intoimpacting contact with said member;

a hammer return hydraulic system including means for alternatelydelivering hydraulic fluid pressure against the hammer for moving itendwise against the compressed gas spring, for retracting the hammer andstoring energy in the compressed gas spring, and releasing saidhydraulic fluid from the hammer, so that the stored energy in thecompressed gasspring can drivethe hammer through its power stroke; and

means for isolating the hammer from the force of said spring meansbefore the hammer reaches the end of itspower stroke, so that the hammeris free to coast to a stop position before reversing its direction oftravel.

30. A gas spring powered impact tool, comprising:

a linear impact member;

a reciprocating-linear hammer in line with said memher;

a compressed gas spring in line with said hammer, for forcibly drivingsaid hammer endwise through a power stroke, into impacting contact withsaid member; and

a hammer return hydraulic system including means responsive to theposition of said hammer for alternately delivering hydraulic fluidpressure against the hammer for moving it endwise against the compressedgas spring, for retracting the hammer and storing energy in thecompressed gas spring, and releasing said hydraulic fluid from thehammer, so that the stored energy in the compressed gas spring can drivethe hammer through its power stroke, said means releasing hydraulicfluid from the hammer immediately following each retraction of saidhammer, and said means delivering additional hydraulic fluid pressureagainst the hammer, for again retracting the hammer, in response to eachtravel of the hammer through a power stroke.

31. A gas spring powered impact tool according to claim 37, wherein saidhydraulic system includes a valve member having a first end portionwhich is in communication with the inflowing hydraulic fluid andreceives an endwise force therefrom tending to move said valve membertowards a first position in which hydraulic fluid is admitted to saidhammer, a second end portion, biasing force means acting on said secondend portion and tending to move the valve member in the oppositedirection towards a second position in which it blocks flow of hydraulicflow to the hammer and the hydraulic fluid pressure on the hammer isreleased, with said inflowing hydraulic fluid by itself exerting alarger force on said valve member than said biasing force means, andvalve means for directing some of the hydraulic fluid against the valvemember, to exert a force on the valve member acting in the samedirection as the biasing force when the valve member is in its firstposition and the piston is against the compressed gas spring and hascompressed same, with the combined forces of said biasing force and saidhydraulic fluid being larger than the force of said inflowing hydraulicfluid and being sufficient to move said valve member into its secondposition.

32. An impact tool according to claim 30, wherein said means foralternately delivering hydraulic fluid to and exhausting it from thehammer includes valve member movable between a first position in whichthe hydraulic fluid is admitted to the hammer, for moving said hammeragainst said spring means and compressing the latter, and a secondposition in which said fluid is exhausted from the hammer and additionalflow to the, hammer is blocked by said valve member, resulting thespring means endwise through its power stroke.

33. A gas spring powered impact tool, comprising:

a linear impact member;

a reciprocating linear hammer in line with said mema compressedrgasspring in line with said hammer, for forcibly driving said hammerendwise through a power stroke, into impacting contact with said member;I

a hammer return hydraulic system including means responsive to theposition of said hammer for alternately delivering hydraulic fluidpressure against the hammer for moving it endwise against the compressedgas spring, for retracting the hammer and storing energyin thecompressed gas spring, and releasing said hydraulic fluid from thehammer, so that the stored energy in the compressed gas spring can drivethe hammer through its power stroke; and

means for trapping a body of the hydraulic fluid between a portion ofthe hammer and a portion of the housing to provide a fluid stop at theend of the power stroke.

1. A linear reciprocating motor comprising: a housing including anelongated piston cavity defined in part by a transverse wall; anelongated piston in said cavity; spring means exerting a force on saidpiston tending to drive it endwise through said cavity towards saidtransverse wall; hydraulic means for driving the piston endwise throughthe cavity in the opposite direction, in opposition to the force of saidspring means, with said spring means being compressed by said piston asit so moves, said hydraulic means including a pressure surface on saidpiston spaced axially from said transverse wall and in fluidcommunication therewith, and means for delivering a hydraulic fluid tosaid transverse wall and said pressure surface, including control valvemeans comprising a linear reciprocating valve member movable between afirst position in which hydraulic fluid is admitted to the pressuresurface and the transverse wall, for moving the piston against thespring means and compressing the latter, and a second position in whichsuch hydraulic fluid is removed from said pressure surface and flow ofadditional hydraulic fluid to said pressure surface is blocked by saidvalve member, resulting in the force of the hydraulic fluid beingremoved from said piston and said piston being forcibly moved by thespring means endwise through the cavity towards said transverse wall,said valve member having a first end portion which is in communicationwith the inflowing hydraulic fluid and receives an endwise forcetherefrom tending to move said valve member towards its first position,a second end portion, biasing force means acting on said second endportion and tending to move the valve member towards its secondposition, with said inflowing hydraulic fluid by itself exerting a forceon said valve member that is larger than then biasing force, said valvemeans further comprising means for directing some of the hydraulic fluidagainst the valve member, to exert a force on the valve member acting inthe same direction as the biasing force wHen the valve member is in itsfirst position, with the combined forces of said biasing force means andsaid hydraulic fluid being larger than the force of said inflowinghydraulic fluid and being sufficient to move said valve member into itssecond position.
 2. A motor according to claim 32, wherein the springmeans is a body of a compressible fluid sealed within a chamber which isexpansible towards the piston.
 3. A linear reciprocating motorcomprising: a housing including an elongated piston cavity defined inpart by a transverse wall; an elongated piston in said cavity; springmeans exerting a force on said piston tending to drive it endwisethrough said cavity towards said transverse wall; hydraulic means fordriving the piston endwise through the cavity in the opposite direction,in opposition to the force of said spring means, with said spring meansbeing compressed by said piston as it so moves, said hydraulic meansincluding a pressure surface on said piston spaced axially from saidtransverse wall and in fluid communication therewith, and means fordelivering a hydraulic fluid to said transverse wall and said pressuresurface, including control valve means comprising a valve member movablebetween a first position in which hydraulic fluid is admitted to thepressure surface and the transverse wall, for moving the piston againstthe spring means and compressing the latter, and a second position inwhich such hydraulic fluid is removed from said pressure surface andflow of additional hydraulic fluid to said pressure surface is blockedby said valve member, resulting in the force of the hydraulic fluidbeing removed from said piston and said piston being forcibly moved bythe spring means endwise through the cavity towards said transversewall, wherein said valve member is annular and surrounds a portion ofsaid piston, said valve member includes a first end surface which isseated against a portion of said transverse surface when the valvemember is in its second position, and a second end surface which issubjected to the force of the spring means, such force tending to urgethe valve member towards said transverse surface, and said housingincludes passageway means for delivering hydraulic fluid to the firstend surface of said valve member, for exerting a force on said first endsurface which moves the valve member from its second position into itsfirst position.
 4. A motor according to claim 3, wherein said housingincludes an internal tubular guide for said valve member in which saidvalve member moves, means forming a bypass chamber which at leastpartially surrounds the valve member, said valve member including sidewall opening means through which the hydraulic fluid flows from thepiston pressure surface into said by-pass chamber when the valve memberis in its second position and the piston is traveling through its powerstroke.
 5. A motor according to claim 4, wherein said housing includesmeans forming an internal annular seat generally at the entrance to saidexhaust chamber, and said valve member includes an exterior annularclosure portion which seats tightly against said seat when the valvemember is in its first position, and prevents hydraulic fluid fromflowing into the by-pass chamber so that it instead exerts a force onthe pressure surface of the piston.
 6. A motor according to claim 5,wherein the tubular guide for said valve member includes means formingan expansible chamber with said valve member, in surroundingrelationship to said valve member, and said valve member includes apiston ring portion defining a movable wall of said chamber, and saidvalve member includes side wall opening means through which thehydraulic fluid flows from the region of the piston pressure surfaceinto said expansible chamber when the valve member is in its firstposition and the piston is retracted against the spring means, wherebythe hydraulic fluid exerts a force on said piston ring which incombination with the spring means fOrce on the valve member acts to movethe valve member from its first position into its second position.
 7. Amotor according to claim 6, wherein said motor includes passageway meansfor exhausting the fluid from the expansible chamber into the by-passchamber when the valve member is in its second position and the pistonis moving through its power stroke.
 8. A motor according to claim 7,wherein said housing includes a separate cavity for said valve memberwhich is offset from the piston cavity, and said valve member includes afirst end surface which is seated against a portion of said transversesurface when the valve member is subjected to the force of the springmeans, such force tending to urge the valve member towards saidtransverse surface, and said housing includes passageway means fordelivering hydraulic fluid to the first end surface of said valvemember, for exerting a force on said first end surface which moves thevalve member from its second position into its first position.
 9. Amotor according to claim 8, wherein said housing includes means forminga by-pass chamber which at least partially surrounds the valve member,said valve member including side wall opening means through which thehydraulic fluid from the piston pressure surface is exhausted when thevalve member is in its second position and the piston is moving throughits power stroke.
 10. A motor according to claim 9, wherein said housingincludes means forming an internal annular seat generally at theentrance to said by-pass chamber, and said valve member includes anexterior annular closure portion which seats tightly against said seatwhen the valve member is in its first position, and prevents hydraulicfluid from flowing into the by-pass chamber so that it instead exerts aforce on the pressure surface of the piston.
 11. A motor according toclaim 10, wherein the cavity for said valve member includes meansforming an expansible chamber with said valve member, in surroundingrelationship to said valve member, and said valve member includes apiston ring portion defining a movable wall of said chamber, and saidhousing includes passageway means through which the hydraulic fluidflows from the region of the piston pressure surface into saidexpansible chamber when the valve member is in its first position andthe piston is retracted against the spring means, whereby the hydraulicfluid exerts a force on said piston ring which in combination with thespring means force on the valve member acts to move the valve memberfrom its first position into its second position.
 12. A motor accordingto claim 11, wherein said motor includes passageway means for exhaustingthe fluid from the expansible chamber when the valve member is in itssecond position and the piston has moved substantially through its powerstroke.
 13. A linear reciprocating motor comprising: a housing includingan elongated piston cavity defined in part by a transverse wall; anelongated piston in said cavity; spring means exerting a force on saidpiston tending to drive it endwise through said cavity towards saidtransverse wall; hydraulic means for driving the piston endwise throughthe cavity in the opposite direction, in opposition to the force of saidspring means, with said spring means being compressed by said piston asit so moves, said hydraulic means including a pressure surface on saidpiston spaced axially from said transverse wall, and means fordelivering a hydraulic fluid to said surface, including control valvemeans movable between a first position in which the hydraulic fluid isadmitted to the pressure surface, for moving the piston against thespring means and compressing the latter, and a second position in whichthe hydraulic fluid is exhausted from the pressure surface, resulting inthe piston being forcibly moved by the spring means endwise through thecavity towards said transverse wall; and limit means for isolating thepiston from the force of said spring means before the piston reaches thetransverse wall, so that the piston is not driven hard against thetransverse wall by the spring means but is free to coast to a stopposition as it approaches the transverse wall before reversing itsdirection of travel.
 14. A motor according to claim 13, wherein a fluidcontaining chamber exists axially between said transverse wall and saidpiston and the fluid therein provides a cushioning stop for said piston.15. A motor according to claim 13, wherein the spring means is a body ofa compressible fluid sealed within a chamber which is expansible towardsthe piston.
 16. A motor according to claim 15, wherein the limit meanscomprises a barrier for the fluid spring chamber and means for arrestingmovement of said barrier independently of said piston, positioned sothat the barrier is stopped before the piston reaches said transversewall and serves to isolate the spring force from the piston.
 17. A motoraccording to claim 16, wherein a fluid containing chamber exists axiallybetween said transverse wall and said piston and the fluid thereinprovides a cushioning stop for said piston.
 18. A motor according toclaim 16, wherein an axial wall means is connected to said barrier andis in part at least tubular and outwardly bounds at least a portion ofsaid piston.
 19. A motor according to claim 18, wherein a bearing isradially received between said axial wall means and an inner side wallportion of the cavity for guiding said axial wall means.
 20. A motoraccording to claim 15, wherein the stop member for said wall means is amechanical spring.
 21. A motor according to claim 20, wherein an axialwall means is connected to said barrier and is in part at least tubularand extends axially from said barrier in at least partial surroundingrelationship to the near end portion of the piston, and said mechanicalspring is annular in shape and has a central opening for looselyreceiving said axial wall means, and a bearing is radially receivedbetween said axial wall means and an inner side wall portion of thecavity, for guiding said axial wall means.
 22. A motor according toclaim 21, wherein a fluid containing chamber exists axially between saidtransverse wall and said piston and the fluid therein provides acushioning stop for said piston.
 23. A gas spring powered impact tool,comprising: a linear impact member; a reciprocating linear hammer inline with said member; a compressed gas spring in line with said hammer,for forcibly driving said hammer endwise through a power stroke, intoimpacting contact with said member; and a hammer return hydraulic systemincluding means for alternately delivering hydraulic fluid pressureagainst the hammer for moving it endwise against the compressed gasspring, for retracting the hammer and storing energy in the compressedgas spring, and releasing said hydraulic fluid from the hammer, so thatthe stored energy in the compressed gas spring can drive the hammerthrough its power stroke.
 24. An impact tool according to claim 24,including means for trapping a body of the hydraulic fluid being aportion of the hammer and a portion of the housing to provide a fluidstop at the end of the power stroke.
 25. A linear reciprocating motorcomprising: a housing including an elongated piston cavity defined inpart by a transverse wall; an elongated piston in said cavity; springmeans exerting a force on said piston tending to drive it endwisethrough said cavity towards said transverse wall; hydraulic means fordriving the piston endwise through the cavity in the opposite direction,in opposition to the force of said spring means, with said spring meansbeing compressed by said piston as it so moves, said hydraulic meansincluding a pressure surface on said piston spaced axially from saidtransverse wall and in fluid communication therewith, and means fordelivering a hydraulic fluid to said transverse wall and said pressuresurface, including control valve means comprising a valve member movablebetween a first position in which hydraulic fluid is admitted to thepressure surface and the transverse wall, for moving the piston againstthe spring means and compressing the latter, and a second position inwhich such hydraulic fluid is removed from said pressure surface andflow of additional hydraulic fluid to said pressure surface is blockedby said valve member, resulting in the force of the hydraulic fluidbeing removed from said piston and said piston being forcibly moved bythe spring means endwise through the cavity towards said transversewall, and means in said housing defining an expansible chamber forreceiving some of the hydraulic fluid when the piston is at leastpartially retracted against the spring, said means including a movablesurface on said valve member, said movable surface on said valve memberbeing directed such that fluid in said chamber acting on said movablesurface tends to urge the valve member towards its second position. 26.A motor according to claim 25, further comprising means in said housingincluding a portion of said piston for trapping fluid in said chamber,so that said fluid will hold the valve member in its second position,until the piston has moved a predetermined distance through its powerstroke.
 27. A linear reciprocating motor comprising: a housing includingan elongated piston cavity defined in part by a transverse wall; anelongated piston in said cavity; spring means exerting a force on saidpiston tending to drive it endwise through said cavity towards saidtransverse wall; hydraulic means for driving the piston endwise throughthe cavity in the opposite direction, in opposition to the force of saidspring means, with said spring means being compressed by said piston asit so moves, said hydraulic means including a pressure surface on saidpiston spaced axially from said transverse wall and in fluidcommunication therewith, and means for delivering a hydraulic fluid tosaid transverse wall and said pressure surface, including control valvemeans comprising a linear reciprocating valve member movable between afirst position in which hydraulic fluid is admitted to the pressuresurface and the transverse wall, for moving the piston against thespring means and compressing the latter, and a second position in whichsuch hydraulic fluid is removed from said pressure surface and flow ofadditional hydraulic fluid to said pressure surface is blocked by saidvalve member, resulting in the force of the hydraulic fluid beingremoved from said piston and said piston being forcibly moved by thespring means endwise through the cavity towards said transverse wall,valve member having a first end portion which is in communication withthe inflowing hydraulic fluid and receives an endwise force therefromtending to move said valve member towards its first position, and asecond end portion subject to the force of said spring means, with theforce of said spring means tending to move the valve member in theopposite direction towards its second position, with said inflowinghydraulic fluid by itself exerting a larger force on said valve memberand with said valve means further comprising means for directing some ofthe hydraulic fluid against the valve member, to exert a force on thevalve member acting in the same direction as the force exerted by thespring means, when the valve member is in its first position and thepiston is against the spring means and has compressed same, with thecombined forces of said spring means and said hydraulic fluid beinglarger than the force of said inflowing hydraulic fluid and beingsufficient to move said valve member into its second position.
 28. Alinear reciprocating motor comprising: a housing including an elongatedpiston cavity defined in part by a transverse wall; an elongated pistonin said cavity; spring means exerting a force on said piston tending todrive it endwise through saId cavity towards said transverse wall;hydraulic means for driving the piston endwise through the cavity in theopposite direction, in opposition to the force of said spring means,with said spring means being compressed by said piston as it so moves,said hydraulic means including a pressure surface on said piston spacedaxially from said transverse wall and in fluid communication therewith,and means for delivering a hydraulic fluid to said transverse wall andsaid pressure surface, including control valve means comprising a valvemember movable between a first position in which hydraulic fluid isadmitted to the pressure surface and the transverse wall, for moving thepiston against the spring means and compressing the latter, and a secondposition in which such hydraulic fluid is removed from said pressuresurface and flow of additional hydraulic fluid to said pressure surfaceis blocked by said valve member, resulting in the force of the hydraulicfluid being removed from said piston and said piston being forciblymoved by the spring means endwise through the cavity towards saidtransverse wall, and means in said housing including a portion of saidpiston for transferring the hydraulic fluid from said pressure surfaceinto an expansible chamber near the spring end of said piston whichchamber is bounded in part by a movable surface on said piston, saidchamber including outlet means leading to exhaust, so that when thepiston is moved by new hydraulic fluid towards said spring the fluid insaid chamber is forced therefrom by the piston through the outlet meansto exhaust.
 29. A gas spring powered impact tool, comprising: a linearimpact member; a reciprocating linear hammer in line with said member; acompressed gas spring in line with said hammer, for forcibly drivingsaid hammer endwise through a power stroke, into impacting contact withsaid member; a hammer return hydraulic system including means foralternately delivering hydraulic fluid pressure against the hammer formoving it endwise against the compressed gas spring, for retracting thehammer and storing energy in the compressed gas spring, and releasingsaid hydraulic fluid from the hammer, so that the stored energy in thecompressed gas spring can drive the hammer through its power stroke; andmeans for isolating the hammer from the force of said spring meansbefore the hammer reaches the end of its power stroke, so that thehammer is free to coast to a stop position before reversing itsdirection of travel.
 30. A gas spring powered impact tool, comprising: alinear impact member; a reciprocating linear hammer in line with saidmember; a compressed gas spring in line with said hammer, for forciblydriving said hammer endwise through a power stroke, into impactingcontact with said member; and a hammer return hydraulic system includingmeans responsive to the position of said hammer for alternatelydelivering hydraulic fluid pressure against the hammer for moving itendwise against the compressed gas spring, for retracting the hammer andstoring energy in the compressed gas spring, and releasing saidhydraulic fluid from the hammer, so that the stored energy in thecompressed gas spring can drive the hammer through its power stroke,said means releasing hydraulic fluid from the hammer immediatelyfollowing each retraction of said hammer, and said means deliveringadditional hydraulic fluid pressure against the hammer, for againretracting the hammer, in response to each travel of the hammer througha power stroke.
 31. A gas spring powered impact tool according to claim37, wherein said hydraulic system includes a valve member having a firstend portion which is in communication with the inflowing hydraulic fluidand receives an endwise force therefrom tending to move said valvemember towards a first position in which hydraulic fluid is admitted tosaid hammer, a second end portion, biasing force means acting on saidsecond end portion and tending to move thE valve member in the oppositedirection towards a second position in which it blocks flow of hydraulicflow to the hammer and the hydraulic fluid pressure on the hammer isreleased, with said inflowing hydraulic fluid by itself exerting alarger force on said valve member than said biasing force means, andvalve means for directing some of the hydraulic fluid against the valvemember, to exert a force on the valve member acting in the samedirection as the biasing force when the valve member is in its firstposition and the piston is against the compressed gas spring and hascompressed same, with the combined forces of said biasing force and saidhydraulic fluid being larger than the force of said inflowing hydraulicfluid and being sufficient to move said valve member into its secondposition.
 32. An impact tool according to claim 30, wherein said meansfor alternately delivering hydraulic fluid to and exhausting it from thehammer includes valve member movable between a first position in whichthe hydraulic fluid is admitted to the hammer, for moving said hammeragainst said spring means and compressing the latter, and a secondposition in which said fluid is exhausted from the hammer and additionalflow to the hammer is blocked by said valve member, resulting in theforce of the hydraulic fluid being removed from said hammer and saidhammer being forcibly moved by the spring means endwise through itspower stroke.
 33. A gas spring powered impact tool, comprising: a linearimpact member; a reciprocating linear hammer in line with said member; acompressed gas spring in line with said hammer, for forcibly drivingsaid hammer endwise through a power stroke, into impacting contact withsaid member; a hammer return hydraulic system including means responsiveto the position of said hammer for alternately delivering hydraulicfluid pressure against the hammer for moving it endwise against thecompressed gas spring, for retracting the hammer and storing energy inthe compressed gas spring, and releasing said hydraulic fluid from thehammer, so that the stored energy in the compressed gas spring can drivethe hammer through its power stroke; and means for trapping a body ofthe hydraulic fluid between a portion of the hammer and a portion of thehousing to provide a fluid stop at the end of the power stroke.